1. Field of the Invention
This invention relates to slope control and current limiting in power integrated circuits.
2. Brief Description of the Prior Art
Generally, in the case of power devices, primarily for automotive applications, it is desirable to control the rate of on and off switching due to EMF problems and RFI problems whereby such signals are not picked up by the automobile radio. This problem has usually been handled using an analog slope controller wherein the rate of change of current and voltage with time (di/dt and dv/dt) is controlled by gradually increasing the gate to source voltage of the power device which in turn gradually reduces the output impedance of the power devices. Analog slope controllers are also often utilized with di/dt sensing elements in the case of power devices that incorporate in the output circuitry recirculation diodes that have long recovery times. Prior art slope control and current limit circuits generally operate by adjusting the gate voltage of the power output device. Analog slope control circuits principally are controlled by an RC time constant, however the capacitance used is the gate capacitance of the power output device. This gate capacitance is not a constant value and varies as the gate/drain voltage varies. Also, the gate capacitance varies in view of the processing, causing the slope control ramp rate to vary. These two factors make it difficult to accurately control the slope in an analog slope control circuit. Such analog circuitry is generally complex, may be incompatible with the process being used to fabricate the integrated circuit or can occupy an undesirably large amount of real estate on the integrated circuit.